Immunoglobulins are responsible for responding to antigenic biochemicals encountered by a host eukaryotic organism. Immunoglobulin binding by a host receptor is important in activating immunological responses to an antigen. As a result, polymorphic receptor variations between individual hosts is expected to have implications in the relative susceptibility of individuals to similar antigenic challenge.
Recent advances in our understanding of the molecular structure and diversity of immunoglobulin receptors have focused attention on opportunities for insights into their role in the pathogenesis of antibody-mediated acute and chronic inflammation.
The IgG binding receptors have been extensively studied. These studies illustrate several aspects of receptor polymorphism that are important in understanding the utility of receptor genotyping individuals. Three distinct families of human receptors for the Fc domain of IgG (FcγRI, FcγRII, and FcγRIII) with different functional potential have been identified (1–3). Within these three FcγR families of related yet diverse molecules, distinct genes and alternative splice variants lead to series of receptor isoforms that have striking differences in the extracellular, transmembrane and intracellular regions. In addition, most receptor isoforms have structurally defined allelic polymorphisms (4–9). The various FcγR isoform; have different functional capacities based on different functional domains and/or sequence motifs. Furthermore, allelic variants of FcγRIIa, FcγRIIIa and FcγRIIIb also have distinct functional capacities (Preliminary Results; 4,9–13). For example, the alleles of FcγRIIa, which differ at amino acid position 131, differ substantially in their capacity to ligate human IgG2 (9,10,12,13) while the F/V-176 alleles of FcγRIIIa differ in their capacity to ligate human IgG1 and IgG3 (4).
Human neutrophils constitutively express FcγRIIa and FcγRIIIb, both of which have functionally significant allelic variants. FcγRIIa has an ITAM signaling motif in the cytoplasmic domain (14), mediates signaling events that are dependent on both phosphorylation and dephosphorylation, and interacts with actin filaments suggesting direct involvement in phagocytosis (15). Although the functional capacity of FcγRIIIb has been more controversial, in a myeloid environment FcγRIIIb initiates a range of cell programs (9,15–17). Importantly, the expression of FcγRIIIb is restricted to neutrophils (and eosinophils). FcγRI expression is inducible on neutrophils by IFNγ or IL-10 (18–20).
Human monocytes constitutively express FcγRIa and FcγRIIa; and with differentiation into monocyte-macrophages, they express FcγRIIIa and FcγRIIb. FcγIa and FcγRIIIa associate with FcεRI γ-chain which has an ITAM signaling motif, and both receptors can initiate degranulation, a respiratory burst and phagocytosis. FcγRIIb is distinct in having an inhibitory ITIM motif (21,22). Most carefully studied on B lymphocytes, FcγRIIb recruits phosphatases (SHP1 and SHIP) to its signaling complex and downregulates net signaling function (23–26).
In contrast to the Fcγ receptors, Fcα receptors are comprised of a single family of molecules most probably derived by alternative splicing from a single gene (27–32). The dominant protein product, FcαRIa (CD89), is a transmembrane protein which is heavily and variably glycosylated (33,34). Like FcγRIa and FcγRIIIa, FcαRIa lacks recognized signaling motifs in its cytoplasmic domain. It associates with FcεRI γ-chain which has an ITAM (35), but unlike FcγRIIIa, FcαRIa does not require the FcεRI γ-chain for expression. Similar to FcγRIa and FcγRIIIa, FcαRIa can initiate degranulation, a respiratory burst and phagocytosis at least in some experimental systems (36–38).
FcαRIa is constitutively expressed on human neutrophils and monocytes with about 6,000–7,000 copies per cell. Its expression is upregulated on activated neutrophils, including exudative neutrophils obtained from the gingival crevice and kidney glomeruli. Among cytokines, TNFα, IL-8 and GM-CSF increase surface expression of FcαRIa while IFNγ and TGFβ downregulate it (39). Alleles of FcαRIa are unexplored.
Immunoglobulin A (IgA) plays a prominent role in host defense at mucosal surfaces and in secretions such as tears, saliva and sweat where secretory IgA is the predominant immunoglobulin isotype. IgA appears to serve as a non-inflammatory regulator of the local immune response through the production of IL1ra and the absence of production of typical pro-inflammatory cytokines (40–44). These effects contrast with the well known anti-microbial defense functions of IgM and IgG which activate cell programs for inflammation and microbial destruction which may, in turn, injure surrounding host tissues. However, it is now recognized that receptors for IgA (FcαR) and for IgG (FcγR) depend upon tyrosine activation motifs for their function and that FcαRI (CD89), FcγRI (CD64) and FcγRIII (CD16a) use the identical FcεRI γ-chain for signaling. Consequently, the critical question of how these γ-chain associated receptors generate such divergent cell programs comes sharply into focus. Indeed, in the gingival lesion of chronic periodontal disease (PD) where both IgAs and IgGs are abundant, the balance of pro-inflammatory and anti-inflammatory programs is critical. Identification of the pivotal anti-inflammatory signaling elements differentially engaged by FcαRI provides a strategy for selective intervention and a target for therapeutic development. IgA through its prominent role in protecting surface tissues against invasion by pathogenic organisms is found throughout the respiratory, gastrointestinal and genitourinary tracts. The progression of diseases associated with these tracts is expected to relate to FcαR function.
Periodontal disease serves as a model for other diseases involving IgA activity. These other diseases include systemic lupus erythematosus (SLE), systemic vasculitis, and IgA nephropathy.
Periodontal disease is a chronic, recurrent inflammatory condition initiated and sustained by subgingival plaque bacteria but defined by the host immune system's inflammatory response which results in destruction of structures supporting the teeth (45–50). While bacterial pathogens and their products can damage host supporting tissues, disease can also arise from an exuberant inflammatory immune response by the host. Normally, this response is self-limited and the organism is eliminated. Some individuals, however, seem more susceptible to persistent or recurrent gingival disease, and this may relate to inherent differences in their immune system response. Indeed, some mechanisms of PD may resemble other chronic inflammatory conditions even though PD's distinctive features depend on the unique characteristics of the gingiva, teeth, supporting structures and microorganisms that reside in the oral cavity.
Among the organisms associated with PD, Porphyromonas gingivalis has been strongly implicated in adult PD (51,52). P. gingivalis-derived proteases damage tissue directly, as well as indirectly through the induction of collagenase secretion and interruption of complement-mediated defenses (53–56). Despite subtle differences in the chemical structure and biological properties of its lipopolysaccharide (LPS) relative to that from enterobacteria, P. gingivalis LPS induces mononuclear phagocytes to release inflammatory cytokines including TNFα, IL-1, and IL-6 (57–60). Fimbriae and other surface components of P. gingivalis also stimulate macrophages to produce IL-1 (61,62). Of the two isoforms of IL-1, IL-1 is prominent in inflamed gingiva (63).
Secretion of IL-1ra by monocytes is a critical anti-inflammatory counterbalance (64). IL-1ra is an analog of IL-1 that blocks the effect of IL-1 by competitively binding to IL-1 receptors without transducing an activation signal. IL-1ra production is induced in monocytes and macrophages by multiple stimuli, IFNγ, IL-4, IL-6, IL-10 and bacterial LPS. IL-1ra production is also stimulated by IgG immune complexes through stimulation of FcγRIIa (CD32) and FcγRIIIa (CD16) (65–67), but the addition of LPS to FcγR stimulated cells augments IL-1β and suppresses IL-1ra production (68). IgA immune complex stimulation of FcαRI (CD89) leads to marked enhancement of IL-1ra production without IL-1 and TNFα (44). Since the ratio of IL-1ra:IL-1 has been implicated in adult periodontitis as well as several other inflammatory diseases including inflammatory bowel disease and systemic lupus erythematosus (69–71), the relative contributions of FcγR and FcαRI in stimulating local neutrophils and monocyte-macrophages is clearly of critical importance.
Recent association studies have suggested that Fcγ receptor polymorphisms (72–73) and that IL-1α/IL-1β genotype (74) may be related to the risk and/or severity of periodontal disease. These insights underscore the importance of the characteristics of the host immune response in defining risk for chronic inflammatory diseases, such as periodontal disease. Since various microorganisms play a role in the etiology of periodontal disease, it is also reasonable to expect that the susceptibility to disease, the severity and response to treatment or resistance would be associated with the MHC polymorphisms which are intimately involved in antigen processing and presenting antigens to T-cells (75). For example, several HLA-A and B alleles are associated with periodontitis among various racial and ethnic groups (76). The strongest association reported was with HLA-B35 (RR=6.03) in West Indies blacks. A similar association was observed between juvenile periodontitis and HLA-A33 and DR2 in African Americans under the age of 30 years (77). In a study of a Japanese family with early-onset periodontitis, high IgG titers were observed to Porphyromonas gingivalis and all subjects had HLA-DR52 and DQI in common (78–79). A study of renal transplant patients from Istanbul who received cyclosporine-A revealed that DR1 was significantly increased in those who did not develop gingival overgrowth compared to those who did (80). However, in Italian transplant patients who received cyclosporin and nifedipine the frequency of HLA-A19 was increased in those with gingival overgrowth compared to those without but did not reach statistical significance (81).
The literature also includes observations that genetic polymorphisms in several FcγR expressed on myeloid cells may influence the recurrence and severity of PD (82,83). These observations raise the possibility that similar polymorphisms in FcαRI may play an important role in the development of PD. FcαRI has several different splice isoforms (27–31) and is extensively, yet variably, glycosylated (33–34).
It would thus be desirable to provide an assay for determining the extent to which FcαRI is also genetically polymorphic, and whether single nucleotide polymorphisms (SNPs) lead to coding changes in both the extracellular and cytoplasmic domains. Characterization of the respective biologies of these SNPs is dependent on an understanding of the functional domains and molecular docking motifs of FcαRI.
It would also be desirable to assay genetically determined and post-translationally modified variations in FcαRI structure and to define the role of both extracellular and cytoplasmic domains essential for functions which may have significant impact on the progression of PD.
It would also be desirable to provide an assay to characterize the signaling elements associated with FcαRI and their role in determining unique FcαRI signaling events.
It would also be desirable to identify novel SNPs affecting FcαRI structure and function.
It would also be desirable to identify both pre- and post-translational variations and modifications of FcαRI and their impact on FcαRI function.
It would also be desirable to establish the role of FcαRI splice variants and of both FcαRI and FcαR polymorphisms in determining the ability of a host to fight certain bacterial and viral infections, the susceptibility to and/or severity of autoimmune diseases, as a prognosis indicator or to identify suitable vaccinations or treatments.